The problem of filling subterranean voids to prevent their collapse and resultant surface subsidence has received acute attention recently, e.g. see the report of the United States Bureau of Mines entitled "Investigation of Subsidence in Rock Springs, Sweet Water Country, Wyoming" by Donner and Whaite.
Several methods have been developed in an attempt to fill mine voids. These methods are generally broken down into two classes. One is called controlled back filling. In this method filler material is carried into the mine void via the tunnels and passages used originally to withdraw mined ores. This method can only be employed where the underground void is accessible to workmen, which in many instances is impossible or at least highly impractical because of cave-ins, flooding and the like.
Blind flushing is the second general method employed. Several techniques of blind flushing have been proposed. The most common method has been to drill an injection hole from the surface of the ground to connect with the void and then to sluice a slurry of particulate material into the void by gravity flow. A conical shaped bed of material is emplaced directly under the borehole and for a very limited lateral distance therefrom. The amount of surface area which gains support depends on the natural angle of repose of the material in air or water, the size of the void, and the depth of the bed. Various materials have been employed in these sluicing methods, e.g. sand, gravel and fly ash. A variation of this method of filling a void is disclosed in U.S. Pat. No. 1,404,112.
U.S. Pat. No. 2,710,332 shows that a void may be filled by injecting solids, e.g. fly ash, in gas suspension. Further refinements of this techinque are disclosed in U.S. Pat. Nos. 3,421,587 and 3,500,934, wherein fly ash or another equivalent very fine particulate materials are blown into the void. The particulate material is very fine, normally of a size such that 90% will pass a 50 mesh screen and 75% will pass a 325 mesh screen.
Another technique for emplacing a particulate material is disclosed in U.S. Pat. No. 3,440,824. In this method a slurry of solid material and water is pumped downwardly through a conduit inserted in a borehole and the slurry is physically directed towards a second borehole by means of a variable direction nozzle attached to the lower end of the conduit and extending into the cavity. Excess slurrying liquid, e.g. water, is pumped outwardly through the second borehole to create a current between the two boreholes which it is alleged aids in distributing the solid material in the void. The present invention would be useful in the method disclosed in U.S. Pat. No. 3,440,824 to assist in distributing the solid material between the first borehole and the second borehole.
The disposal of waste spent shale is taught by U.S. Pat. No. 3,459,003. Therein a slurry composed of a major part of the shale and water is pumped into the void. A cement formed from the remaining shale is then pumped into the void -- to fill "at least a portion of the void therein." The shale particles used are to be within the size range 5 microns to 0.5 inch; larger waste particles are crushed to this size.
The above-described methods generally represent the known techniques which are employed in an attempt to prevent subsidence caused by underground cavities and voids. All of these methods, however, suffer from some disadvantage. First, the radial distance around the borehole which can be essentially completely filled is relatively limited. Secondly, it is usually difficult to substantially fill the void to the ceiling. Thirdly, many boreholes must be provided when the void to be filled extends over a great distance. This latter disadvantage is particularly troublesome when the void is located beneath a populated area since structures, streets and the like prevent the drilling of a necessary number of boreholes. For example, it has been reported in the Bureau of Mines Report, cited previously, that in Rock Springs, Wyo. if a blind sluicing method was employed as many as 3000 boreholes would be required (as many as 75 in a single square block area) to treat 200 acres of land. Even with this many boreholes the voids cannot be completely filled and support is provided only under the streets, alleys and other areas of public access. Only a very limited amount of support for structures, e.g. dwellings and industrial plants can be provided.
A common cause of these disadvantages is the inability to distribute the filler solids in the void at sufficient distance from the input injection shaft. It has been discovered that this is partially caused by absorption of the liquid in which the filler solids are suspended by the solids which are deposited near the exit of the injection shaft. A plug or bridge will form close to the entry point, causing the liquid to flow through the deposit rather than over it. Flow over the deposit is necessary to transport the suspended solids to the extremity of the deposit. Here the solids drop out and extend the deposit as the liquid spreads over the surface of the deposit and the flow rate decreases.
The use of a particular size distribution of particles has been suggested to plug pores and to form thin filter cakes on the internal surfaces of well shafts. U.S. Pat. No. 2,815,079 teaches the use of pre-sized hard granular particles to seal the plug where mud filtercakes are formed. This patent recommends that a certain maximum particle size should be employed and in addition that certain smaller sizes should be employed in specific percentages.
The approach of U.S. Pat. No. 3,042,608 is to use resinoids to seal openings. Emphasis is placed on the advantageous properties of resinous substances for pore plugging purposes. A recommended size distribution is suggested as a corollary consideration once the choice of a resinoid has been made.
The problem presented in filling mined out voids is almost the opposite of that solved by the two above inventions. When filling mined out voids the particles must be held in suspension until they flow sufficiently far from the input point. The cited references must plug pores very close to the input point -- the solid may not remain suspended for very long, or else the plugging of the pores will not occur. The present invention concerns the discovery that mined out voids can be completely filled over large radial distances by employing a fill material having a certain permeability.